The present invention relates to a fuel cell power generation system for use in a portable power source, electric car power source, cogeneration system etc. and more particularly relates to a fuel cell power generation system using polymer electrolyte membranes.
As a typical example of fuel cells, polymer electrolyte fuel cells are known. In a known polymer electrolyte fuel cell, an anode and a cathode are formed so as to sandwich a polymer electrolyte membrane therebetween, and the anode and the cathode are respectively supplied with fuel gas containing hydrogen and oxidizing gas containing oxygen such as air (hereinafter, the fuel gas and oxidizing gas are sometimes collectively referred to as “reactive gas”). While hydrogen ions are created, in the anode, through the liberation of electrons from the hydrogen atoms contained in the fuel gas caused by the electrode reaction, these electrons reach the cathode after passing through the external circuit (load). Meanwhile, the hydrogen ions reach the cathode after passing through the polymer electrolyte membrane. In the cathode, the hydrogen ions, the electrons and the oxygen contained in the oxidizing gas are combined, thereby producing water. During this reaction, electric power and heat are generated at the same time.
As the polymer electrolyte membrane, a perfluorocarbon sulfonic acid material is used. Since this polymer electrolyte membrane elicits ion conductivity when it contains moisture, the reactive gas is usually humidified and then supplied to the fuel cell.
Excessive humidification of the reactive gas may, on one hand, cause flooding because water is generated in the cathode. On the other hand, it is desirable to supply the reactive gas that has been humidified so as to have a relative moisture of 100%, because the ion conductivity of the polymer electrolyte membrane needs to be increased in order to improve the performance of the fuel cell. In addition, it has been found that if the reactive gas having a dew point equal to or lower than the operative temperature of the fuel cell, the electrolyte made of perfluorocarbon sulfonic acid decomposes so that fluoride ions liquate out from the polymer electrolyte membrane, resulting in degradation of the polymer electrolyte membrane.
With the intention of increasing the service life of the fuel cell by inhibiting the degradation of the polymer electrolyte membrane, there have been made several attempts to perform the so-called “full humidification operation” in which the fuel cell is supplied with the reactive gas having the dew point equal to cell temperature, while preventing the flooding (e.g. The 8th FCDIC Fuel Cell Symposium Proceedings (pp 61-64) (hereinafter referred as nonpatent literature 1) (see the operating conditions stated in the captions of FIGS. 3, 4)).
Another known fuel cell system is such that the degradation of the polymer electrolyte membrane and flooding are inhibited by providing the electric cells with gas passage and/or electrode configuration which varies from cell to cell and distributing reactive gas having the same degree of humidification etc. to the respective electric cells (e.g., Japanese Patent Gazette No. 3596332 (hereinafter referred as patent literature 1)).